The present invention relates to a device for measuring the thermal conductivity of a fluid, to be used in particular for the detection of compounds present in the same fluid.
The identification of the chemical composition of a substance raised to the gaseous or vapor state is usually done by gas chromatography. In such apparatus, a gaseous mixture, or a mixture of gas and vapor, containing the sample to be analyzed and an inert carrier gas, is made to flow, after an elution process, to a detector that measures variations in certain physical characteristics of the same gaseous mixture.
Devices that measure variation in the thermal conductivity of a fluid are widely used as substance analyzers in gas chromatography apparatus. Such devices detect variations in the conductivity of the gaseous mixture with which they are supplied, and give values that relate to the composition of the same gaseous mixture.
Detectors that measure the thermal conductivity of fluids consist schematically of a metal filament located inside a duct through which the gaseous mixture to be analyzed is made to flow. The metal filament, whose electrical resistance is temperature dependent, is heated by an electric current and the heat generated is transmitted to the inside walls of the duct mainly by conduction through the gaseous mixture.
Variations from one instant to the next in the thermal conductivity of the mixturexe2x80x94due to changes in the chemical composition of the samexe2x80x94cause variations in the flow of heat dissipated by the filament. Such variations in the heat flow lead to variations in the temperature of the filament, and accordingly, in its electrical resistance. The thermal conductivity of the gaseous mixture can be measured simply by measuring the electrical resistance of the metal filament as the gaseous mixture passes through the duct.
The practical implementation of such schemes, however, has encountered serious problems, such as drift in the voltage that controls the current of the metal filament and serious inaccuracies in the results when even small variations occur in the temperature of the mixture to be analyzed or in the temperature of the walls of the duct. A device of simple construction widely employed to overcome the aforementioned problems comprises an electrical measurement bridge (such as, for instance, a Wheatstone bridge) in which the two pairs of heat-sensitive resistance elements, diagonally opposed with respect to supply and detection, are placed in two respective ducts. Only carrier gas is made to flow in one of the ducts, while a mixture of carrier gas and sample to be analyzed is made to flow in the other, in such a way that the conductivity of the carrier gas alone constitutes the reference value to measure the conductivity of the carrier gas and sample mixture.
If the voltage across the terminals of the detector diagonal of the bridge when only carrier gas is made to flow in the duct is considered as the reference value, for instance, each variation of the voltage with respect to this reference value, observed in correspondence to the entry of the mixture to be analyzed into one of the ducts, will depend only on the nature and concentration of the substances present in the mixture.
In fact, any variation of the temperature of the carrier gas that occurs in both ducts, will not influence the detection. Nor will the latter be even affected by variations of the temperature of the ducts, since these, not being insulated thermally, are placed on a single support (defined below as the body of the detector) that is maintained at constant temperature.
One drawback not eliminated by similar embodiments is the short useful life of the resistance filaments placed in contact with the gas flow, due to their rapid oxidation and their contact with any corrosive compounds present in the gaseous mixture. The filaments need to be replaced frequently, but they are very expensive because of the requirement for characteristics of low reactivity and linearity of the voltage-temperature response. In particular, the known embodiment described above provides for frequent and simultaneous replacement of all four filaments.
Furthermore, the body of the detector, which supports two ducts and four filaments and must include a means of thermoregulation, is not very small and permits neither convenient use in reduced spaces nor the support to be held at constant temperature.
The device according to the invention comprises a first section with at least one duct for the passage of a fluid whose thermal conductivity is to be measured and at least one heat-sensitive resistance element placed within that duct, and a second section for the detection of an electrical quantity which is dependent on the heat-sensitive resistance element. There are first means of thermoregulation to maintain the first section at a constant temperature T1, and second means of thermoregulation to maintain the second section at a constant temperature T2 independent of the temperature T1 of the first section. According to a preferred embodiment of the invention, the device comprises a first section having two ducts in which there are located respectively two heat-sensitive resistance elements and a second section that forms an electrical measurement bridge with the resistance elements present in the first section. The fluid whose thermal conductivity is to be measured and a reference fluid respectively are made to flow in the two ducts. Furthermore, the two sections are maintained at independent temperatures T1 and T2 by their respective means of thermoregulation.
A preferred embodiment of the invention will be described by way of example and not of limitation, with reference to the figures attached, in which: